A conventional semiconductor device such as a solar cell has a structure in which a plurality of photovoltaic elements are arranged on a substrate and they are connected to each other in series as shown in Japanese Published Patent Application 59-167072, Japanese Published Patent Application 59-182578, and Japanese Published Patent Application 61-50381.
Conventional techniques for making a photovoltaic device using a semiconductor thin film will be described in detail hereinafter. FIGS. 14(a) to 14(f) are schematic sectional views showing manufacturing steps of the conventional semiconductor device using a semiconductor thin film, which is described in the above Japanese Published Patent Application 59-182578.
In FIG. 14(f), reference numeral 101 designates an insulating substrate. A first electrode layer 113a is selectively formed on the substrate 101. A semiconductor layer 115a formed of a semiconductor thin film is formed on the electrode layer 113a. A second electrode layer 117a is formed on the semiconductor layer 115a. The semiconductor layer 115a and the electrode layers 113a and 117a form one unit photovoltaic element.
Next, its manufacturing method will be described.
Referring to FIG. 14(a), the first electrode layer comprising ITO (indium tin oxide), tin oxide or the like is formed on the insulating substrate 101 of glass. Then, referring to FIG. 14(b), the first electrode layer 113 is divided into predetermined patterns by irradiation with a laser beam. Then, referring to FIG. 14(c), semiconductor thin films are formed thereon as the semiconductor layer 115. In this case, the semiconductor layer 115 is formed by depositing a thin film of amorphous silicon, microcrystalline silicon, an alloy of amorphous silicon and carbon or the like. For example, thin films of p type amorphous silicon, i type amorphous silicon and n type amorphous silicon are used. Then, referring to FIG. 14(d), some portions of the semiconductor layer 115 are removed by means such as laser beam irradiation or etching according to the pattern of the first electrode 113a thereby forming the openings 115b. Then, referring to FIG. 14(e), the structure is covered with the second electrode layer 117 of metal such as aluminum. Then, referring to FIG. 14(f), the structure is processed by such as a laser beam to form the openings 117b.
As described above, there is provided a structure in which the semiconductor layer 115a is provided at each of element regions on the insulating substrate 101, that is, at each of regions on which a unit photovoltaic element is to be formed and this semiconductor layer 115a is sandwiched by the first electrode layer 113a and the second electrode layer 117a. The first electrode layer 113a under the semiconductor layer 115a at each region is connected to the second electrode 117a on the semiconductor layer 115a at the adjacent region.
According to the photovoltaic element of the above structure, if the semiconductor layer 115a has a structure which consists of p type, i type, and n type amorphous silicon thin films, when light irradiates the semiconductor layer 115a through the insulating substrate 101 of glass, the semiconductor layer 115a an generates electromotive force. This electromotive force is generated in the direction of the film thickness so that the p type film may have a positive potential and the n type film may have a negative potential. In addition, since the electrode of the n type side of the semiconductor layer 115a of the unit photovoltaic element is connected to the electrode on the p type side of the semiconductor layer 115a of the adjacent element, the electromotive force generated at the semiconductor layer 115a of each element is added, whereby a high voltage can be generated as a whole. In addition, the amount of current generated can be adjusted by the area of the semiconductor layer 115a of each element.
When the semiconductor layer 115a is formed of a material such as amorphous silicon which can be formed at a relatively low temperature of approximately 300.degree. C. at most, materials of the insulating substrate 101, the first electrode layer 113a and the second electrode layer 117a and their processing means can be selected from a wide range. As a result a structure in which the semiconductor layers 115a arranged on the insulating substrate 101 at predetermined intervals are connected in series by the electrode layers 113a and 117a can be easily obtained as described above.
However, when noncrystalline silicon is used, the conversion efficiency of incident light is converted to electrical energy is low and the conversion efficiency degrades with light exposure. Therefore, crystalline silicon is preferably used as the material of the semiconductor layer 115a.
Meanwhile, when the semiconductor layer 115a is formed of crystalline silicon, processing at a high temperature from approximately 600.degree. C. to 1400.degree. C. which is close to the melting point of silicon has to be performed. In this case the insulating substrate 101, the first electrode layer 113 and the second electrode layer 117 should be formed of materials which can endure the high temperature, so that the choice of materials is limited. In addition, a constituent of the material could diffuse into the silicon crystal as an impurity during processing at a high temperature, which could degrade the performance of the semiconductor device. Thus, it is difficult to produce the above integrated structure using crystalline silicon.
Meanwhile, some efforts have been made to provide a photovoltaic semiconductor device by forming a thin film of crystalline silicon on a substrate. For example, FIGS. 15 and 16 are views showing the structures of semiconductor devices shown in the Conference-Record of the 20th IEEE Photovoltaic Specialists Conference. In FIGS. 15 and 16, reference numeral 111 designates a high-temperature-resistant substrate. A diffusion blocking layer 102 is formed on the substrate 111. A semiconductor layer 105 is formed on the whole surface of the substrate 111 and the blocking layer 102. An emitter layer 106 is formed on the semiconductor layer 105. A metal electrode 107 is formed on the surface.
As the high-temperature-resistant substrate 111, a low purity polycrystalline silicon substrate is used in the structure shown in FIG. 15 and a conductive ceramic is used in the structure shown in FIG. 16, respectively.
According to the conventional example shown in FIG. 15, the diffusion blocking layer 102 comprising a silicon dioxide film is formed on the high-temperature-resistant substrate 111 and then the openings 102a are formed therein in order to electrically connect the high-temperature-resistant substrate 111 to the semiconductor layer 105. More specifically, the high-temperature-resistant substrate 111 serves as an electrode for extracting the current which is generated when light irradiates the semiconductor thin film 105. The semiconductor layer 105 comprising a polycrystalline silicon thin film is formed thereon by chemical vapor deposition method. Since the polycrystalline silicon thin film formed by the chemical vapor deposition method consists of small crystal grains, this is once melted by infrared heating and then recrystallized. Thereafter, the emitter layer 106 is formed on the surface by impurity diffusion and then the metal electrode 107 is formed.
According to this structure, the blocking layer 102 prevents an impurity which could degrade the performance of the semiconductor device from diffusing from the high-temperature-resistant substrate 111 into the semiconductor layer 105 during the recrystallization and also reflects light which enters the semiconductor layer 105 but is not completely absorbed in it and then introduces the light to the semiconductor layer 105 again. However, since the opening 102a is formed in the blocking layer 102, impurities contained in the high-temperature-resistant substrate 111 diffuse into the semiconductor layer 105 through the opening 102a when the semiconductor layer 105 is recrystallized, with the result that the performance of the semiconductor device is degraded.
In addition, according to the conventional example shown in FIG. 16, the blocking layer 102 is formed on the whole surface of the heat resistant substrate 111 comprising a conductive ceramic and then the polycrystalline silicon thin film is formed thereon as the semiconductor thin film 105 by the solution growth method using tin as a solvent. Then, similar to FIG. 15, the emitter layer 106 and the metal electrode 107 are formed.
In this example, although the material of the blocking layer 102 is not specified, the blocking layer 102 should be formed of a conductive material in order to electrically connect the semiconductor thin film 105 to the high-temperature-resistant substrate 111 as in FIG. 15. Therefore, it should be formed of metal or the like. In general, when a metal impurity group enters into the silicon semiconductor, it exerts bad influences on electrical characteristics of the semiconductor of silicon. Meanwhile, when the semiconductor layer 105 is formed on the blocking layer 102 by the solution growth method, the growth temperature must be a high temperature of around 1000.degree. C. Therefore, in this case also, the impurities contained in the material of the blocking layer 102 diffuse into the semiconductor layer 105 while the semiconductor layer 105 is formed, degrading performance of the semiconductor device.
In addition, according to the conventional semiconductor devices shown in FIGS. 15 and 16, since the high-temperature-resistant substrate 111 is used as an electrode for extracting the current generated in the semiconductor layer 105, the high-temperature-resistant substrate 111 has to be formed of a conductive material. Since the high-temperature-resistant substrate 111 is uniformly connected to the semiconductor layer 105 over the whole surface, it is substantially impossible to implement the integrated structure shown in FIG. 14 in which the unit photovoltaic elements are connected in series.
Since the conventional semiconductor device is constructed as described above, it is difficult to implement an integrated structure using a semiconductor material such as crystalline silicon. In addition, impurities diffuse into the semiconductor thin film from the substrates or the metal layers which contact the semiconductor film.